Adipocytes and osteoblasts may share a common precursor residing in the bone marrow. This is suggested by clinical observations and animal research models which demonstrate an inverse relationship between bone marrow adiposity and bone density. Elucidation of the mechanisms governing how bone marrow stromal cells differentiate into adipocytes and osteoblasts may carry implications for metabolic diseases such as obesity and of bone diseases such as osteoporosis. Our studies focus on the potential role in this mechanism of Gsa, a heterotrimeric G protein subunit that mediates cyclic AMP-dependent signaling of G protein-coupled receptors including the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor (PPR). PPR signaling is believed to promote osteoblast differentiation and may also suppress adipocyte differentiation. These actions may be mediated in part by canonical Wnt signaling. Studies from this laboratory revealed that mice with targeted deletion of Gsa early in the osteoblast lineage (Osterix-Cre:Gsa(fl/fl)) exhibit osteoporosis and multiple fractures at birth along with increased bone marrow adiposity. This laboratory previously reported that Osterix-Cre:Gsa(fl/fl) mice have increased expression of sclerostin, an inhibitor of canonical Wnt signaling secreted by osteocytes, and decreased gene expression of Wnt target genes. Based on these observations, we hypothesized that Gsa expressed in cells of the early osteoblast lineage has a role in regulation of osteoblast and adipocyte differentiation, and that this role was mediated in part by canonical Wnt signaling. Preliminary studies using in vitro colony forming unit assays suggested that targeted deletion of Gsa early in the osteoblast lineage leads to fewer numbers of total progenitor cells and osteoprogenitor cells, but does not change the number of adipocyte progenitors. The question remains, however, whether the increased adipogenic potential of cells derived from Osterix-Cre:Gsa fl/fl mice is accounted for by increased adipocyte progenitor number or conversion of Gsa null osteoblasts into adipocytes. The experiments of Specific Aim I address this question by using the the double fluorescent reporter gene membrane-targeted tandem dimer tomato/green fluorescent protein to identify adipocytes that expressed Osterix:Cre and thus presumably were originally of the osteoblast lineage. The experiments of Specific Aim II address the potential role of Wnt signaling in how Gsa signaling in early osteoblasts regulates adipocyte and osteoblast differentiation. Firstly, these experiments will study whether there is decreased Wnt activity in Osterix-Cre:Gsa mice. Secondly, we will study whether Gsa signaling in early osteoblasts interact Wnt signaling upstream of b-catenin stabilization. This question will address by testing the effects of in utero lithium exposure on Osterix-Cre:Gsa fl/fl mice upon bone mass, bone marrow adiposity, and relative numbers of adipocyte progenitors and osteoprogenitors. PUBLIC HEALTH RELEVANCE: This study focuses on mechanisms underlying how fat cells and bone cells develop from the bone marrow. This may help improve understanding and treatment of diseases such as obesity and osteoporosis.